The present invention relates to electrorheological fluids. More specifically, it relates to a method for increasing and/or modulating the yield shear stress of electrorheological fluids and to an apparatus employing such method.
Electrorheological (ER) fluids and ER effects are well known in the art. Since the discovery of ER fluids around 1947, many efforts have been made to increase the yield shear stress of ER fluids to a level at which they can advantageously be used for various industrial applications, such as actuators for torque transmission (such as clutch, brake, and power transmission), vibration absorption (such as shock absorber, engine mount, and damper), fluid control (such as servo valve and pressure valve) and many other industrial applications. ER fluids are generally more energy efficient than hydraulic, mechanical or electromechanical devices which serve the same function. However, the strength of ER fluids has not been generally high enough in the past. The search for strong ER fluids has produced limited results. ER fluids currently have yield shear stress up to about 5 kPa in the presence of an applied electric field, not generally sufficient for major industrial applications, most of which therefore do not utilize ER fluids. The present invention achieves increased yield shear stress through a novel use of the microstructure properties of ER fluids.
The flow characteristics of an ER fluid change when an electric field is applied through it. The ER fluid responds to the applied electric field by what can be described as progressively gelling. More specifically, the ER fluid is generally comprised of a carrier fluid, such as pump oil, silicone oil, mineral oil, or chlorinated paraffin. Fine particles, such as polymers, minerals, or ceramics, are suspended in the carrier fluid. When an electric field is applied through the ER fluid, positive and negative charges on the particles separate, thus giving each particle a positive end and a negative end. The suspended particles are then attracted to each other and form chains leading from one electrode to the other. These chains of particles cause the ER fluid to xe2x80x9cgelxe2x80x9d in the electric field between the electrodes in proportion to the magnitude of the applied electric field. Thus, the prior art provides a means to increase the yield shear stress (xe2x80x9ceffective viscosityxe2x80x9d) of an ER fluid by application of an electrical field, but the maximum yield shear stress thus attained (up to about 5 kPa) is still not sufficient for use in most industrial applications.
For the reasons described above, a method for increasing and/or modulating the yield shear stress of ER fluids by a simple process would be desirable. In addition an apparatus employing such method of increasing and/or modulating the yield shear stress of ER fluids would further be desirable.
The present invention is directed to a method for increasing and/or modulating the yield shear stress of ER fluids and to an apparatus employing such method.
According to the method of the present invention, a sufficient electric field is first applied to the ER fluid to cause particles within the ER fluid to form into chains of particles and to cause the ER fluid to xe2x80x9cgelxe2x80x9d in the electric field applied between the electrodes. Then, a sufficient pressure is applied to the ER fluid between the electrodes, while the electric field applied in the previous step is substantially maintained. This causes the chains of particles to thicken and thus increases the yield shear stress. The pressure and the shear stress may be applied in any direction, relative to that of the applied electric field, which causes the chains of particles to thicken. When the increased shear stress is no longer needed or needs to be modulated upwardly or downwardly, the pressure and, optionally, the electric field are adjusted upwardly or downwardly as required.
In a first embodiment of the method of the invention, the pressure is applied in a direction substantially perpendicular to that of the electric field, in which case the chains aggregate and thus become thicker. In a second embodiment, the pressure is applied in a direction substantially parallel to that of the electric field, in which case the chains become shorter and thus become thicker. However, as contemplated in the present invention, the pressure may be applied in any direction with respect to that of the applied electric field which results in thickening of the chains through a combination of shortening and aggregation of the chains.
According to the apparatus of the present invention, the ER fluid is placed between and in communication with two working structures, between which a force or a torque is to be transmitted (through the ER fluid). The ER fluid is also in communication with at least two electrodes having different electric potentials, which serve to apply an electric field through the ER fluid when an increase in the yield shear stress is desired. The electrodes may be on the same or different working structures, or be separate from them. A sufficient electric potential is first applied to the electrodes to cause particles within the ER fluid between the electrodes to form into chains of particles and to cause the ER fluid to gel. Then, a sufficient pressure is applied to the ER fluid, suitably by bringing the two working structures closer together, while the electric potential applied in the previous step is substantially maintained, to cause the chains of particles to become thicker and thus to increase the yield shear stress. The increase in the yield shear stress resulting from the applied pressure causes any force or torque which is provided by one working structure to be transmitted more efficiently to the other working structure. When the more effective force or torque transmission is no longer needed, the pressure and, optionally, the electric field may be removed. If an intermediately effective force or torque transmission is needed, the applied pressure may be decreased while the applied electric field remains applied at the same, a higher, or a lower level. Thus, once a higher yield shear stress has been established, it may be modulated upwardly or downwardly as required by increasing or decreasing the strength of the electric field, the applied pressure, or both.
In a first embodiment of the apparatus of the invention, the first working structure is preferably electrically insulating, but may also be grounded electrically, and the electrodes are all on the second working structure, the working surface of which is parallel to the working surface of the first working structure. In this embodiment, the chains of particles form in the vicinity of the second working structure, between electrodes through the ER fluid. Pressure is applied in a direction perpendicular to that of the electric field by bringing the two working surfaces closer together, which causes aggregation of the chains into thicker chains, providing an increase in the yield shear stress. In one variation of this embodiment, the electrodes have an alternating arrangement on the second working structure, separated by insulating zones, so that neighboring (adjacent) electrodes have different electric potentials. However, other electrode arrangements are possible with similar results. In addition, other variations of this embodiment are possible in which the two working structures are not parallel and/or not planar.
In a second embodiment of the apparatus of the invention, the two working structures are parallel and each one serves as an electrode. In this embodiment, the chains of particles form between the two working structures, through the ER fluid. Pressure is applied in a direction parallel to that of the electric field by bringing the two working surfaces closer together, which causes the chains to become shorter and thus thicker, again providing an increase in the yield shear stress. Variations of this embodiment are also possible in which the two working structures are not parallel and/or not planar.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but not restrictive, of the invention.